The present invention is concerned with an improved process for the oxidative carbonylation of alcohols to produce dialkyl oxalates. More particularly, this invention comprises the reaction of a mixture of carbon monoxide and oxygen with an alcohol in the presence of a heterogeneous catalyst comprising palladium and thallium.
The preparation of oxalate esters by the reaction of carbon monoxide and alcohol is well known. U.S. Pat. No. 3,393,136 describes a process for the preparation of oxalates by contacting carbon monoxide at superatmospheric pressure, with a saturated monohydric alcohol solution of a platinum group metal salt and a soluble ferric or cupric salt (redox agent) while maintaining the salts in a highly oxidized state by the simultaneous introduction of oxygen or the application of a direct current electrical potential to the reaction zone. Water scavengers or dehydrating agents such as alkyl orthoformic acid esters must be added to the liquid phase to prevent the accumulation of water.
In an article by Donald F. Fenton and Paul J. Steinwand, Journal of Organic Chemistry, Vol. 39, No. 5, 1974, pp. 701-704, a general mechanism for the oxidative carbonylation of alcohols to yield dialkyl oxalates using a palladium redox system, oxygen and dehydrating agents has been proposed. In the absence of the necessary dehydrating agent, a large amount of carbon dioxide is formed and oxalates are not produced. The necessity of the iron or copper redox system during the oxalate synthesis is emphasized. West German Pat. No. 2,213,435 discloses a method for the synthesis of oxalic acid or oxalate esters using water or alcohol, respectively. A platinum group metal salt, a salt of a metal more electropositive than the platinum group metal, e.g. copper (II) chloride, and an alkali metal salt such as lithium chloride comprise the catalyst. Oxygen in stoichiometric amounts was employed as the oxidant. A disadvantage of such reaction is that explosive mixtures of oxygen and carbon monoxide are necessary to effect reaction. Alcohol conversion of less than 5 percent is obtained. Under non-explosive conditions, only trace amounts of oxalate can be obtained.
U.S. Pat. No. 3,994,960 describes a process for the production of dialkyl oxalates by reacting an aliphatic alcohol with CO and oxygen under pressure in the presence of a catalyst of a mixture of a salt of a metal from the platinum group and a salt of copper or iron and a reaction accelerator including nitrates, sulfates, bicarbonates, carbonates, tertiary amines and hydroxides and carboxylates of alkali metals and alkaline earth metals, pyridine, quinoline, urea and thiourea. Conversion of the alcohol employed to the dialkyl oxalates in such a process is low, generally less than 9 mol percent.
In a process similar to that of U.S. Pat. No. 3,994,960 above, West German Offenlegungschrift No. 2,601,139 shows the production of oxalic acid or its alkyl esters by reacting aliphatic alcohols or water with oxygen and carbon monoxide in the presence of palladium salts, redox salts and an amine or ammonia base.
U.S. Pat. Nos. 4,005,128 and 4,005,129 are concerned with the oxidative carbonylation of alcohols with carbon monoxide carried out in the presence of a stoichiometric quantity of a metal oxide such as copper or iron, and a catalytic amount of a metal such as palladium, platinum, copper, etc., and in the presence of an amine or an amine plus an amine salt, respectively.
U.S. Pat. No. 4,005,130 is concerned with a process for the preparation of oxalate esters by the oxidative carbonylation of alcohols with carbon monoxide in the presence of a catalytic amount of copper, nickel, cadmium, cobalt or a zinc metal salt catalyst and at least a stoichiometric amount of an unsubstituted or halogen-substituted 2,5-cyclohexadiene-1,4-dione(1,4-benzoquinone). High yields and selectivity of the oxalate ester, over the carbonate ester and CO.sub.2, are obtained and maximized by regulating temperature, carbon monoxide pressure and metal salt catalyst and by maintaining substantially anhydrous conditions.
U.S. Pat. No. 4,076,949 claims a process for the preparation of oxalate esters by reacting an alcohol with a mixture of carbon monoxide and oxygen in the presence of a catalytic mixture of:
(a) a palladium, rhodium, platinum, copper, or cadmium metal salt compound or mixture thereof; PA1 (b) an aliphatic, cycloaliphatic, aromatic or heterocyclic amine or ammmonium; PA1 (c) a copper (I), copper (II), iron (II) or iron (III) oxidant salt compound; and PA1 (d) an ammonium or substituted ammonium salt compound or acid with a counterion other than a halide. Alternatively, a ligand or coordination complex compound of the metal salt compound may be employed.
U.S. Pat. No. 4,118,589 relates to a process for producing oxalic acid and esters of oxalic acid. More particularly, this patent describes a catalytic process for preparing oxalic acid and esters of same by the oxidative reaction, in a liquid phase, of carbon monoxide and water or alcohols with oxygen in the presence of redox systems. The catalyst systems used in accordance with the teaching of the patent comprise a redox catalyst consisting essentially of a salt of Pd (II) and salts of a metal more electropositive than Pd having at least two oxidation states and, optionally, salts of alkaline metals, and cocatalytic amounts of at least one base having the formula R.sub.2 N in which the groups R, which may be like or unlike, are selected from the group consisting of hydrogen and alkyl radicals having from 1 to 10 carbon atoms.
U.S. Pat. No. 4,138,587 describes a process for the preparation of dialkyl oxalates by reacting an aliphatic alcohol with carbon monoxide under pressure in the presence of a catalyst comprising a platinum group metal or a salt thereof, an accelerator composed of one or more compounds selected from the group consisting of nitric acid and nitrogen oxides, and molecular oxygen.
U.S. Pat. No. 4,230,881 discloses a method for preparing esters of oxalic acid by the reaction of carbon monoxide and an alcohol in the presence of a catalyst consisting of a complex of palladium and a co-catalyst consisting of a compound having acidic properties selected from the group consisting of amine salts, carboxylic acids, phenol, succinimide and phthalimide.
Commonly assigned U.S. Pat. No. 4,281,174 is concerned with a process for the preparation of dialkyl oxalates by the oxidative carbonylation of alcohols which comprises reacting a mixture of carbon monoxide and oxygen with an alcohol in the presence of a catalytic amount of a catalyst comprising palladium in complex combination with a ligand, a quinone and a redox agent.
The liquid oxalate esters are solvents, but the preferred use is as feedstock for hydrogenation to ethylene glycol. The oxalate esters can also be hydrolyzed to oxalic acid which has a number of industrial uses.